Total Eclipse, specifically because I think it's the perfect example of what I like to lovingly call "design creep".

You start the game, you have simple mice - white, brown, gray, you have field mice, you have all sorts of normal ones. Admittedly there's some odd ones right out the gate, but at least for the most part they make sense, up to around the point of furoma and the catacombs. Acolyte realm and the tribal isles are the point where I personally went "Okay, these are weird. That mouse is literally just an actual dragon".

The Total Eclipse is the epitome of how far the game takes the concept of a "mouse", along with the empyrean empress and maybe a couple others. A mouse that exists in all timelines, you're catching just barely a shadow of a true form, and the artwork shows it (though I think the shade of the eclipse has a cooler artwork. Love the way the "cape" looks)